1. Field of the Invention
The present invention relates to an inductor device and a process of production of the same.
2. Description of the Related Art
The market is constantly demanding that electronic equipment be made smaller in size. Greater compactness is therefore required in the devices used in electronic equipment as well. Electronic devices originally having lead wires have evolved into so-called xe2x80x9cchip devicesxe2x80x9d without lead wires along with the advances made in surface mounting technology. Capacitors, inductors, and other devices comprised mainly of ceramics are produced using the sheet process based on thick film forming techniques or using screen printing techniques etc. and using cofiring process of the ceramics and metal. This enables realization of a monolithic structure provided with internal conductors and a further reduction of size.
The following process of production has been adopted to produce such a chip-shaped inductor device.
First, a ceramic powder is mixed with a solution containing a binder or organic solvent etc. This mixture is cast on a polyethylene terephthalate (PET) film using a doctor blade method etc. to obtain a green sheet of several tens of microns or several hundreds of microns in thickness. Next, this green sheet is machined or processed by laser etc. to form through holes for connecting coil pattern units of different layers. The thus obtained green sheet is coated with a silver or a silver-palladium conductor paste by screen printing to form conductive coil pattern units corresponding to the internal conductors. At this stage, the through holes are also filled with the paste for the electrical connection between layers.
A predetermined number of these green sheets are then stacked and press-bonded at a suitable temperature and pressure, then cut into portions corresponding to individual chips which are then processed to remove the binder and sintered. The sintered chips are barrel polished, then coated with silver paste for forming the terminations and then again heat treated. These are then electrolytically plated to form a tin or other coating. As a result of the above steps, a coil structure is realized inside of the insulator comprised of the ceramic and thereby an inductor device is fabricated.
There have been even further demands for miniaturization of such inductor devices. The main chip sizes have shifted from the 3216 (3.2xc3x971.6xc3x970.9 mm) shape to 2012 (2.0xc3x971.2xc3x970.9 mm), 1608 (1.6xc3x970.8xc3x970.8 mm), and even further smaller shapes. Recently, chip sizes of 1005 (1xc3x970.5xc3x970.5 mm) have been realized. This trend toward miniaturization has gradually made the requirements for dimensional accuracy (clearance) on the steps severer in order to obtain stable and high quality.
For example, in an inductor device of a chip size of 1005, the stack deviation of the internal conductor layers is not allowed to exceed more than 30 xcexcm. If this is exceeded, remarkable variations occur in the inductance or impedance. In extreme cases, the internal conductors are even exposed.
In the case of an inductor device of a relatively large chip size of the related art, this stack deviation was not serious enough to have a notable effect on the properties of the device, but with a chip size of about 1005, stack deviations have a tremendous effect on the device properties.
In the inductor devices of a relatively large size of the related art, the coil pattern units of the internal conductors in the different layers were L-shaped or reverse L-shaped. The L-shaped pattern units and reverse L-shaped pattern units were alternately stacked and through holes were provided at the ends of these patterns to connect the patterns of the different layers. The starting ends and finishing ends of the coil formed in this way were connected to leadout patterns.
Experiments by the present inventors etc. have shown, however, that when making the coil pattern units of the internal conductors at different layers L-shaped and reverse L-shaped and simply making the coil pattern units smaller in order to obtain a 1005 or other small-sized inductor device, the stack deviation of the internal conductors remarkably progresses.
The reason why the stack deviation progresses in a small-sized inductor device is believed to be as follows: That is, to obtain a predetermined inductance or impedance despite reduction of the chip size, it is necessary to increase the number of turns of the coil. Therefore, it is necessary to make each of the ceramic layers thinner. Further, a low resistance is required in the internal conductors, so it is not allowed to make the conductors thinner by the same rate as the ceramic sheet. Therefore, a smaller chip size results in a remarkable non-flatness of a green sheet after printing.
As a result, when applying pressure to superposed green sheets to form them into a stack, the conductor portions, which are relatively hard compared with the green sheets themselves, interfere with each other and therefore cause remarkable stack deviation. In particular, in a printing pattern based on the L-shapes of the related art, the stacked green sheets were pushed at a slant 3-dimensionally through the internal conductorsxe2x80x94which only aggravated the stack deviation. This phenomenon became a major hurdle to be overcome for stabilization of the quality of the device along with the increased reduction of the chip size of the devices.
Various proposals have been made to solve this problem. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-77074 discloses to press printed green sheets in advance in order to flatten them. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-192945 discloses to give the ceramic sheets grooves identical with the conductor patterns in advance, print the conductor paste in the grooves, and thereby obtain a flat ceramic sheet containing conductors. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-192955 discloses not to peel off the PET film from the ceramic sheet, but to repeatedly stack another ceramic sheet, press it, then peel off the film. This method uses the fact that PET film undergoes little deformation and as a result could be considered a means for preventing stack deviation. Further, Japanese Unexamined Patent Publication (Kokai) No. 6-20843 discloses to provide a plurality of through holes along the circumference of the printed conductors so as to disperse the pressure at the time of press-bonding.
Each of the methods disclosed in the above publications added further steps to the method of stacking the ceramic sheets of the related art or made major changes in it. Further, they were more complicated than the method of the related art and therefore disadvantageous from the viewpoint of productivity.
An object of the present invention is to provide an inductor device able to suppress stack deviation without complicating the production processxe2x80x94even if the device is made smallerxe2x80x94and a process for the production of the same.
The present inventors engaged in intensive studies of a small-sized inductor device able to suppress stack deviation without complicating the production process and a process for production of the same and as a result discovered that it is possible to suppress the stack deviation by suitably determining the pattern shape of coil pattern units formed between insulator layers of the device and thereby completed the present invention.
According to the present invention, there is provided an inductor device formed comprising a plurality of insulating layers; conductive coil pattern units each formed between insulating layers, having two substantially parallel linear patterns and a curved pattern connecting first ends of the linear patterns, and having a ratio A1/A2, where the total of the areas of the two linear patterns seen from the plane view is A1 and the area of the curved pattern seen from the plane view is A2, of 1.45 to 1.85, preferably 1.55 to 1.75, more preferably 1.62 to 1.68; and connection portions formed at second ends of the linear pattern and connecting upper and lower coil pattern units separated by the insulating layers in a coil-shape.
When A1/A2 is smaller than 1.45, the areas of the linear patterns are too small compared with the area of the curved pattern and as a result the sectional area of the coil becomes smaller and there is a tendency for a sufficient inductance not being able to be secured. When A1/A2 is larger than 1.85, the areas of the linear patterns are too large compared with the area of the curved pattern and stack deviation tends to easily occur in the direction substantially perpendicular to the longitudinal direction of the linear patterns.
In the present invention, preferably, when the total area of a unit section of the insulating layer in which one coil pattern unit is contained is A0, the ratio (A1+A2)/A0 is in a range of 0.10 to 0.30, preferably 0.13 to 0.20, more preferably 0.15 to 0.17.
When the ratio (A1+A2)/A0 is smaller than 0.10, the areas of the coil unit patterns for constituting the coil are too small compared with the area of the insulating layer and the DC resistance becomes too large, so this is not preferred. When the ratio (A1+A2)/A0 is larger than 0.30, the sectional area of the coil becomes smaller and there is a tendency for the required inductance not to be able to be secured.
In the present invention, when the line width of the linear patterns is W1 and the radius of curvature of the outer circumference of the curved pattern is R, preferably the ratio W1/R is in a range from 1/2 to 4/5 , more preferably 3/5 to 2/3.
When the ratio W1/R is smaller than 1/4 , the line width of the linear patterns is too narrow and stack deviation tends to easily progress. This is believed to be due to the fact that if the line width of the linear patterns is narrow, when a linear pattern positioned at an upper layer and a linear pattern positioned at a lower layer are superposed, stack deviation easily occurs in the direction substantially perpendicular to the longitudinal direction of the linear patterns. Further, when the ratio W1/R is larger than 4/5 , the diameter of the curved pattern becomes smaller and the line width of the patterns becomes thicker, so the diameter of the coil obtained inside the device becomes smaller and there is a tendency for the desired inductance property not to be able to be secured.
In the present invention, two coil pattern units positioned above and below an insulating layer are preferably arranged at line symmetric positions with respect to a center line dividing the insulating layer across the longitudinal direction as seen from the plane view. By arranging them in this way, it is possible to obtain an inductor device with little stack deviation while obtaining the desired inductance characteristic.
Alternatively, the coil pattern units are preferably line symmetric patterns about a center line dividing the insulating layer across the width direction seen from a plane view. By using such patterns, it is possible to obtain an inductor device with little stack deviation.
In the present invention, two or more coil pattern units may be arranged between insulating layers. By arranging a plurality of coil pattern units in this way, it is possible to obtain an inductor array device having a plurality of coils inside a single device.
According to the present invention, there is provided a process for the production of an inductor device comprising the steps of forming a green sheet to form an insulating layer; forming on the surface of the green sheet a conductive coil pattern unit having two substantially parallel linear patterns and a curved pattern connecting first ends of the linear patterns and having a ratio A1/A2, where the total of the areas of the two linear patterns seen from the plane view is A1 and the area of the curved pattern seen from the plane view is A2, of 1.45 to 1.85; stacking a plurality of green sheets formed with the coil pattern units and connecting the upper and lower coil pattern units separated by the green sheets through through holes to form a coil shape; and sintering the stacked green sheets.
The process of production according to the present invention may include, before the sintering step, a step of cutting the stacked green sheets into pieces each containing one coil pattern unit.
Alternatively, the process of production according to the present invention may include, before the sintering step, a step of cutting the stacked green sheets into pieces each containing a plurality of coil pattern units.
According to the process of production according to the present invention, it is possible to obtain an inductor device able to suppress stack deviation without complicating the production process even if the device is made small in size.